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2002 RXTE IAU Circulars RXTE

With the kind permission of Dr Brian Marsden of the International Astronomical Union's Central Bureau for Astronomical Telegrams, the full text of IAU circulars concerning RXTE is reprinted here - a week after their original promulgation by the IAU.

Please note that the text below includes only the parts of the IAUC relevant to RXTE, and that some IAUC contain more than one bulletin about RXTE.

7897, 7893, 7888, 7886, 7880, 7870, 7867, 7856, 7843, 7831, 7792,

May 9, 2002 - 7897

XTE J0929-314

D. K. Galloway, E. H. Morgan, R. A. Remillard, and D. Chakrabarty, Massachusetts Institute of Technology, write: "Observations of the recently discovered 185-Hz accreting millisecond pulsar XTE J0929-314 (cf. IAUC 7888, 7889, 7893, 7895) with RXTE/PCA on May 9.5 UT indicate that the pulsar is still active at a flux level of about 20 mCrab (2-10 keV), down from 30 mCrab on May 2. Orbital Doppler shifts of the pulse frequency were clearly detected. Further RXTE observations are scheduled over the next few days. Multiwavelength follow-up observations are strongly encouraged."

May 7, 2002 - 7893

XTE J0929-314

R. A. Remillard, Massachusetts Institute of Technology; and J. Swank and T. Strohmayer, Goddard Space Flight Center, NASA, report the discovery of 185-Hz pulsations in XTE J0929-314 (IAUC 7889). This source was observed in a brief pointed observation with RXTE on May 2, while the average flux was 28 mCrab (2-30 keV). A power spectrum was computed for 800 s of PCA data, and a highly significant pulsation is seen at 185.09 Hz, with a strong harmonic at 370.18 Hz. This is the third known pulsar in which pulsations faster than 10 ms can be seen in the persistent x-ray emission. Rasters across the source give an improved position R.A. = 9h29m18s, Decl. = -31o23'.1 (equinox J2000.0; systematic uncertainty 1'), consistent with the optical candidate (IAUC 7889).

M. P. Rupen, V. Dhawan, and A. J. Mioduszewski, National Radio Astronomy Observatory, report the detection of a radio counterpart to the x-ray transient XTE J0929-314 (IAUC 7888). Observations with the Very Large Array (VLA) at 4.86 GHz show a source with flux density 0.31 +/- 0.07 mJy on May 3, and 0.36 +/- 0.05 mJy on May 7, at R.A. = 9h29m20s.194, Decl. = -31o23'03".41 (equinox J2000.0; uncertainty +/- 0".3). This is 0".8 from the optical position reported by Greenhill et al. (IAUC 7889) and provides strong evidence that this optical identification is correct. Further optical and x-ray observations are strongly encouraged.

P. Cacella, Brasilia, Brazil, reports that an unfiltered CCD image taken with a 0.25-m reflector shows a variable (mag 18.3) that is possibly the optical counterpart to XTE J0929-314 at position end figures 20s.22, 03".6.

April 30, 2002 - 7888

XTE J0929-31

R. A. Remillard, Massachusetts Institute of Technology (MIT), for the RXTE ASM Team at MIT and Goddard Space Flight Center, reports the discovery of a faint x-ray transient at R.A. = 9h29m22s, Decl. = -31o22'.8 (equinox J2000.0; estimated uncertainty 3', 90- percent confidence). The source is visible in sky maps computed from 6-day intervals of ASM data. Average fluxes (2-12 keV): Apr. 13-18, 15 +/- 2 mCrab; Apr. 19-24, 20 +/- 2; Apr. 25-30, 26 +/- 1. The error circle contains no noteworthy sources in the Simbad catalogues. The ASM hardness ratios suggest an x-ray spectrum similar to transients associated with weakly magnetic neutron stars or some blackhole systems. Optical and radio observations are encouraged.

April 29, 2002 - 7880


M. F. Corcoran, Universities Space Research Association and Goddard Space Flight Center, reports: "Recent observations of this peculiar luminous star with the Proportional Counter Array on the Rossi X-ray Timing Explorer show that the star has undergone an especially strong x-ray flare in the band 2-10 keV. The flare started on Mar. 12, apparently reached x-ray maximum on Apr. 21, has not yet declined to the pre-flare brightness level. The peak x-ray flux in the and 2-10 keV is 1.8 x 10**-10 erg s**-1 cm**-2; eta Car has not been this bright in x-rays since 1997 May 29 (i.e., about 7 months prior to the x-ray eclipse)."

April 20, 2002 - 7880

XTE J1901+014 and XTE J1751-305

R. A. Remillard, Massachusetts Institute of Technology (MIT); and D. A. Smith, University of Michigan, for the RXTE ASM Team at MIT and NASA Goddard Space Flight Center, report the discovery of a fast x-ray transient. The detection was initially thought to be a gamma-ray burst (cf. http://gcn.gsfc.nasa.gov/gcn/gcn3/1332.gcn3), with a peak flux of 0.9 Crab (2-12 keV). The event began on Apr. 6.76268 UT, lasting > 2 min and < 3.15 hr (see http://xte.mit.edu/xtej1901+104). The ASM data show rapid evolution to a very hard spectrum during this outburst, but the IPN instruments failed to detect the source above 15 keV (K. Hurley, private communication). Furthermore, archival ASM data show a previous outburst at the same position on 1997 June 21.215, which was longer than 6 min and < 8 hr, with a peak flux of 0.4 Crab. The combined ASM observations yield a best position R.A. = 19h01m36s, Decl. = +1 26'.4 (equinox J2000.0; estimated uncertainty 3' in R.A. and 2' in Decl.). The error box contains one very faint, unidentified x-ray source from the Einstein Galactic Plane Survey: 2E 1859.1+0122 (R.A. = 19h01m40s.2, Decl. = +1 27'13", equinox J2000.0; error radius 48"; Hertz and Grindlay 1988, A.J. 96, 233). The time scales for these eruptions are reminiscent of the microquasar and blackhole binary V4641 Sgr, so optical and radio observations of XTE J1901+014 are strongly encouraged.

XTE J1751-305 Z. Wang and D. Chakrabarty, MIT, report that the candidate infrared counterpart that they proposed for XTE J1751-305 (IAUC 7874) is now likely excluded. Further analysis of their J-band images of the field from Apr. 9 UT have led to a revised astrometric solution (0".5 rms) that places the candidate outside the 0".6-radius Chandra x-ray error circle (IAUC 7876), lying 0".8 west and 0".4 north of its center. No stars are detected within the error circle (limiting mag J = 20.5). The two detected stars closest to the Chandra position are the former candidate (0".9 distant) and one (J = 18.0 +/- 0.5; blended with a neighbor to the southwest having J = 13.5) that is 1".2 distant.

C. B. Markwardt, University of Maryland and Goddard Space Flight Center, reports that XTE J1751-305 (IAUC 7867) has become undetectable by the RXTE PCA instrument. Since discovery, RXTE monitoring has shown a decrease in the x-ray flux, and as of Apr. 17.5 UT the 2-10-keV upper limit was approximately 1 mCrab (due primarily to other nearby sources).

April 08, 2002 - 7870

XTE J1751-305

C. B. Markwardt, University of Maryland and Goddard Space Flight Center (GSFC); and J. H. Swank, GSFC, report that the orbital period of the recently discovered pulsar XTE J1751-305 (IAUC 7867) is 2545.3(1) s. RXTE PCA observations taken intermittently during Apr. 4.64-7.54 UT show doppler modulation of the pulsed signal, with a projected semimajor axis of 10.06(6) light-ms. The barycentric mean spin frequency of the pulsar is 435.31812(4) Hz, and the epoch of 90-deg mean longitude is JD 2452369.1389(1). The mass function of the pulsar is 1.26 x 10**-6 solar mass, which gives a minimum mass for the companion of 0.014 solar mass (approximately 15 Jupiter masses). No x-ray eclipses or absorption dips were observed, indicating that the source is not viewed directly edge-on. A power-law fit to the x-ray spectrum has a photon index of 1.9. The neutral-hydrogen-absorption column density for a range of spectral models is 1 to 3 x 10**22 cm-2, indicating that the optical counterpart will be heavily absorbed. The x-ray flux is variable at the 1-to-2-percent level (r.m.s.), primarily on the timescale of several orbital periods. The average x-ray flux has declined from 56 to 43 mCrab, corresponding to an exponential-decay constant of 10 days, thus justifying prompt follow-up observations.

April 04, 2002 - 7867

XTE J1751-305

XTE J1751-305 C. B. Markwardt, University of Maryland and Goddard Space Flight Center (GSFC); and J. H. Swank, GSFC, report that RXTE PCA monitoring observations of the galactic-center region on Apr. 3.6 UT revealed a new millisecond pulsar, designated XTE J1751-305. The best fitting source position is R.A. = 17h51m.5, Decl. = -30o30' (equinox 2000.0), with an estimated position uncertainty of about 5', due to nearby confusing sources (see item by in 't Zand et al., below). In the first 200 s of a pointed RXTE PCA observation on Apr. 4.65, x-ray pulsations were detected (with 10-sigma significance) at a barycentered frequency of 435.313 Hz +/- 0.005 Hz. The r.m.s. pulsed amplitude in the band 2-10 keV was 2.5 percent. The approximate 2-10-keV x-ray fluxes on Apr. 3.6 and 4.6 were 55 +/- 1 and 60 +/- 2 mCrab, respectively. More observations by RXTE are scheduled. This is only the second accretion-powered millisecond pulsar. Further observations at other wavelengths are encouraged.

J. J. M. in 't Zand, Astronomical Institute, Utrecht University, and Space Research Organization Netherlands (SRON); P. Lowes, SRON and BeppoSAX Science Operation Center, Rome; J. Heise, SRON; and A. Bazzano and P. Ubertini, Istituto di Astrofisica Spaziale e Fisica Cosmica, Rome, report that XTE J1751-305 was detected in a BeppoSAX Wide Field Camera observation during Apr. 4.3-4.6 UT at a flux of 52 +/- 3 mCrab (2-28 keV) and localized at R.A. = 17h51m16s, Decl. = -30o37'.5 (equinox 2000.0; uncertainty 1'.2, 99-percent confidence). No systematic flux trends were observed at more than the 10-percent sensitivity, and no x-ray bursts were detected.

March 21, 2002 - 7856

XTE J1908+094

P. M. Woods, C. Kouveliotou, M. H. Finger, and E. Gogus, National Space Science and Technology Center; and J. Swank, C. Markwardt, and T. Strohmayer, Goddard Space Flight Center, report the serendipitous discovery of a new x-ray transient, XTE J1908+094, in RXTE PCA observations of the soft-gamma-ray repeater SGR 1900+14, triggered following the burst activity on Feb. 17-18 (GCN 1253). These observations failed to detect the 5.2-s SGR pulsations, pointing towards a possible new source as the origin of the high x-ray flux. An RXTE PCA scan of the region around SGR 1900+14 on Feb. 21 was consistent with emission only from known sources (and no new sources). However, the scans required SGR 1900+14 to be 20 times brighter than its quiescent flux level (GCN 1256). A Director's Discretionary Time Chandra observation on Mar. 11 showed that the SGR was quiescent and did not reveal any new source within the Chandra ACIS field-of-view. A subsequent RXTE PCA scan on Mar. 17, taken in combination with the first scan, required that a new source be included in the fit. The best-fit position is R.A. = 19h08m50s, Decl. = +9 22'.5 (equinox J2000.0; estimated 2' systematic error radius), or approximately 24' away from the SGR source. The source spectrum (2-30 keV) can be best fit with a power-law function including photoelectric absorption (column density N_h = 2.3 x 10**22, photon index = 1.55). Iron line emission is present, but may be due to the Galactic ridge. Between Feb. 19 and Mar. 17, the source flux (2-10 keV) has risen from 26 to 64 mCrab. The power spectrum is flat between 1 mHz and 0.1 Hz, falling approximately as 1/f**0.5 up to 1 Hz. At 1 Hz is seen a broad quasiperiodic oscillation peak and a break to a 1/f^2 power law, which continues to 4 Hz. The fractional rms amplitude from 1 mHz to 4 Hz is 43 percent. No coherent pulsations are seen between 0.001 and 1024 Hz. The authors conclude that XTE J1908+094 is a new blackhole candidate.

March 06, 2002 - 7843

SAX J1805.5-2031

P. Lowes, National Institute for Space Research, Space Research Organization Netherlands (SRON), Utrecht, and BeppoSAX Science Operation Center (BSOC), Rome; J. J. M. in 't Zand, Astronomical Institute, Utrecht University, and SRON; J. Heise, SRON; M. Cocchi and L. Natalucci, Istituto di Astrofisica Spaziale e Fisica Cosmica, Rome; G. Gennaro and M. Stornelli, BSOC, report that on Mar. 5.5-5.9 UT, Wide Field Camera unit 1 on BeppoSAX detected a new x-ray transient, designated SAX J1805.5-2031 and located at R.A. = 18h05m34s, Decl. = -20o30'.8 (equinox 2000.0; uncertainty 1'.8, 99-percent confidence). The flux was 0.04 Crab units in the range 2-28 keV. No x-ray bursts were detected.

C. B. Markwardt, University of Maryland and Goddard Space Flight Center (GSFC); and E. Smith and J. Swank, GSFC, report that the source SAX J1805.5-2031 was observed by the RXTE PCA at 2-10 keV in monitoring scans of the Galactic bulge region around Mar. 2.97 UT (flux 51 +/- 2 mCrab) and in a 3100-s pointed PCA observation starting Mar. 5.84 (flux 36 +/- 1 mCrab). The position of the source in the PCA scans is consistent with the BeppoSAX WFC position (cf. Lowes et al., above). The 2-20-keV spectrum is well fitted with a combination of blackbody (kT = 0.9 keV) emission and a power law (photon index -2.9) for a column density of 4 x 10**22 cm**-2. No strong pulsations of quasiperiodic oscillations are detected; the total fractional r.m.s. variability of the source is 16 percent in the band 0.001-2048 Hz. Additional RXTE observations are planned.

February 16, 2002 - 7831


J. H. Swank and E. A. Smith, Goddard Space Flight Center, report: "Rossi X-Ray Timing Explorer observations indicate that Aquila X-1 has begun an x-ray outburst. The ASM reported a level of 33 mCrab during Feb. 12-15. On Feb. 15.6 UT, the flux began a rise at the rate of 8.3 ASM counts/s (122 mCrab) per day. The PCA watched as it rose from 75 to 120 mCrab during Feb. 16.03-16.25. One x-ray burst was observed at the level of 1 Crab. Additional RXTE observations are scheduled."

January 15, 2002 - 7792

XTE J1550-564

XTE J1550-564 J. Swank, E. Smith, and C. Markwardt, Goddard Space Flight Center, report: "Rossi X-Ray Timing Explorer observations indicate that XTE J1550-564 is again at a flux of 50-100 mCrab, which it reached in 2001 Jan. for about 3 weeks. Since 2001 Oct., the ASM has indicated intermittent levels of a few ASM counts/s. In a pointed observation on 2002 Jan. 10.875 UT, the PCA confirmed a level 60 mCrab, with a very hard spectrum (power-law index 1.4 +/- 0.2, cutoff 30-60 keV), corresponding to a 2-50-keV luminosity of 3 x 10**36 erg/s at 2500 pc. The spectrum includes Fe K-shell line emission, possibly in a reflection component, and about 10 percent of the luminosity is in a component attributable to a disk. High- amplitude fluctuations (33-percent rms) are again present, with characteristic times of 20-100 s (cf. IAUC 7575). Based on recent behavior, a full outburst is not likely; nevertheless, additional RXTE observations are scheduled and coordinated observations at other wavelengths are encouraged."

C. Bailyn, Yale University, adds: "YALO observations on Jan. 12.319 and 13.337 UT yield V = 18.36 and 18.33, respectively. These values are near the peak optical flux from the last two outbursts of this source."

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